Polyamide resins are well known as a class of resins, as are numerous methods for their preparation. Polyamide resins are typically manufactured by reacting a di- or polyfunctional amine with a di- or polyfunctional acid. Most of the commonly-employed diacids and diamines yield polyamide resins which are essentially linear.
The properties of polyamide resins will vary considerably, depending upon the particular synthetic reactants employed. Polyamide resins which are prepared from relatively short chain diacids and diamines having, for example, 5-10 carbon atoms will tend to be relatively crystalline and have excellent fiber forming properties. These types of polyamide resins are typically referred to as nylons.
Polyamide resins are also prepared from relatively long chain polyfunctional acids and diamines. A particularly important class of polyamide resins of this type are referred to as polymerized fatty acid polyamide resins. The polymerized fatty acid polyamide resins are especially useful in products such as hot melt adhesives, water resistant coatings, and binders for printing inks, because of their physical properties, including high strength, excellent flexibility, water and solvent resistance, and the ability to form smooth, nontacky coatings and films.
The polyfunctional acids used in the preparation of polymerized fatty acid polyamide resins are derived from higher molecular weight unsaturated fatty acids by polymerization. In the polymerization process, the fatty acids having double bond functionalities combine to produce mixtures of higher molecular weight polymeric acids.
The polymerized fatty acid polyamide resins are, in turn, typically prepared by reacting one or more suitable diamines--most commonly relatively short chain diamines--with the polymerized fatty acid. Often, another diacid is also reacted to increase the softening point, tensile strength, or other properties. The polymerized fatty acid polyamide resins which are obtained tend to be more amorphous than the nylon types of polyamides resins and are generally more flexible. The differences in the physical properties of the polymerized fatty acid polyamide resins as compared to the nylon types of polyamide resins are related to the long chain length and structural variations of the polymerized fatty acid component.
One of the problems encountered with the polyamide resins--particularly the polymeric fatty acid polyamides--relates to the methods used to apply the resins to substrates. One method which has been used involves heating the polyamide resins above their melting point and then applying the molten resins to the substrate. This technique, however, has certain inherent problems. For example, polyamide resins typically have high melting points, often higher than the distortion temperatures of the substrates onto which they are to be applied. Accordingly, the hot melt method can only be used in certain limited applications which require relatively expensive application equipment. Thus, the use of molten polyamide resins is not practical in applications such as, for example, printing and coating. Molten polyamide resins are also impractical where the resin is to be applied as a latent hot melt layer to be activated at a later time.
It has been recognized that certain of the problems associated with the polyamide resins might be solved if the polyamides could be applied at ambient temperatures as solutions or dispersions. For many applications, however, solutions of polyamide resins are unsatisfactory. Polyamide resins as a class have excellent resistance to solvents; even with respect to those solvents in which the polyamide resins are soluble, the solubility typically is relatively low. Furthermore, the solvents which have been used to make polyamide resin solutions often adversely react with the substrates to which the polyamide resin solutions are applied. Further problems associated with solvent solutions are that most solvents used are relatively expensive, often difficult or impossible to remove from the applied coatings, and present fire, toxicity, and environmental pollution problems.
To overcome or at least reduce the problems associated with such solvent-based systems, it has been suggested to prepare emulsions or dispersions of the polyamide resins in water. Early emulsions were prepared by initially dissolving the polyamide resin in an organic solvent and then using selected emulsification agents to form an emulsion of the solvent solution and water. However, the resulting solvent/water/polyamide resin emulsions still had the problems associated with the presence of solvents and were relatively unstable. Those skilled in the art will appreciate that instability is manifested in aqueous resin emulsions or dispersions by phenomena such as phase separation, creaming, coalescence, flocculation, or gelation. Films formed from solvent-containing emulsions also tended to have an undesirable tackiness.
In British patent 1,491,136 there was disclosed a method for forming aqueous dispersions of various plastic powders, including polyamide resin powders. In the disclosed method, the polymer resin was first mechanically reduced to a powder form and then blended with water and a thickening agent. The method was less than satisfactory, The mechanical reduction of the resins to the required particle size was both expensive and difficult to control, especially for flexible polymers, and often caused thermal degradation of the polymers. Furthermore, the resulting thickened dispersions had limited utility in many applications because of the relatively high viscosity due to the thickening agent.
It is also known to render a polyamide resin more readily dispersible in water by chemically modifying the resin so as to include solubilizing groups. This includes, for example, incorporating alkoxymethyl groups, as disclosed in U.S. Pat. No. 2,430,860 (Carirns) and U.S. Pat. No. 2,714,075 (Watson, et al.). However, the incorporation of the additional groups into the polyamide resin increases the cost of the polymer and also typically reduces the desirable properties of the polyamide resins, especially in relation to water and solvent resistance.
Another known method for increasing the water dispersibility of polyamide resins involves formation of a resin having a considerable excess of either free carboxyl or free amine groups. At least a portion of the free acid or free amine groups are then neutralized to form salt groups on the polyamide resin, which salt groups act as internal surfactants to facilitate the dispersion of the modified polyamide in water. In U.S. Pat. No. 2,811,459 (Wittcoff, et al.) there is disclosed a method for preparing polymerized fatty acid polyamide dispersions wherein the polyamide is formed from a substantial excess of a diamine. The resulting polyamide resins are then dispersed in an aqueous solution of an acid so that the acid forms ammonium salt groups which act as internal surfactants which allow formation of an aqueous dispersion. In U.S. Pat. No. 2,768,090 (Wittcoff, et al.) a similar process is disclosed wherein the excess amine groups of a polyamide resin are reacted with an acid to form intrinsic ammonium salt groups and, hence, a cationic dispersion which is converted to an anionic dispersion by charge inversion. A similar salt forming process utilizing free amino groups was disclosed in U.S. Pat. No. 2,824,848 (Wittcoff). In U.S. Pat. No. 2,926,117 (Wittcoff) there is disclosed a method wherein the polyamide resin formed with a deliberate excess of acid groups is then dispersed in an aqueous medium containing an alkaline substance to cause formation of carboxylate salt groups which act as internal surfactants.
The discussed methods for preparing aqueous dispersions of polymerized fatty acid polyamides having salt groups are relatively effective in initially forming aqueous dispersions. However, the dispersions have limited stability and are not satisfactory for use in many applications, as their synthesis requires the presence of substantial amounts of free acid or free amino groups which adversely effect the performance properties of the dispersed polyamide resin. Optimal properties are typically achieved by conducting the amidations so as to cause as complete as a reaction as possible. This requires that approximately stoichiometric amounts of the starting diacid and diamine be employed and that the reaction be conducted so as to produce a final product having a low amine number and low acid number. The presence of substantial excesses of either reactant or an incomplete reaction--as required for the prior art salt forming polyamide material--inherently reduces the chain length and the resulting strength and flexibility of the polyamide resin.
Furthermore, incorporation of polymers having substantial excess amounts of unreacted polymerized fatty acids typically results in unstable materials. The fatty acids can be liberated from the polymer and cause exceptional tackiness and undesirable degradation of the desired properties of the polyamide resin. These polyamide resins continue to react during application, which causes increases in molecular weight and coating viscosity, as well as changes in the melting point. A still further problem encountered with the method wherein the salt forms of the polyamide resins are used is that the salts tend to decompose during application and the resulting material becomes undesirably tacky when applied. This is particularly undesirable in many applications, such as in printing inks and protective coatings.
Certain of the problems associated with aqueous polyamide resin dispersions can be obviated by the methods disclosed in U.S. Pat. No. 4,886,844 (Hayes) for the preparation of stable aqueous dispersions of nonsolvated, un-neutralized, polymerized fatty acid polyamide resins having low acid and amine number. As disclosed therein, molten resin, water, and a surfactant are subjected to sufficient comminuting forces to form an emulsion in which resin droplets have a volume average size distribution of about 20 microns or less.
However, even the aqueous polyamide resin dispersions according to Hayes are not without problems attendant to their use. For example, these aqueous dispersions can be drawn into films, but must be heated to within about 10.degree. C. of the resin's softening point for clear, continuous films to properly form. For example, in U.S. Pat. No. 557,649 (Wittcoff), the use of polyamide suspensions in heat-seal compositions requires a minimum temperature of 70.degree. C. Thus, it would be more desirable if such films could be formed at lower temperatures, preferably ambient temperatures. This is particularly true where resins having relatively high softening points are employed.